This paper experimentally studies the influence of two-phase flow behavior on the operating limits of steam injector (SI). SI is the passive jet pump with a converging-diverging structure. SI operates without the electrical power and discharges water at high pressure. However, in regard to the operating limits of SI, it is unclear which takes into account the discharged flow structure. In this paper, the pressure distributions along the flow direction and the discharge pressure were measured by changing the inlet steam pressure and the load on the exit of the test section. Furthermore, the discharged flow at the diffuser was observed with a high-speed camera. As a result, a boundary was observed in the discharged flow at which the flow structure changed. The area from the throat to this boundary is considered as a two-phase region which steam did not condense completely in the mixing nozzle remained. This boundary moved upstream as the load on the exit increased, and a significant pressure rise occurred at the position where the boundary was reached. In addition, white propagations towards the downstream direction were observed. This propagation is considered as pressure wave propagation. The propagating speed was estimated using image processing. Assuming that the pressure wave propagates at sonic speed, the void fraction at the discharged flow was estimated by the existing model. Based on the above, the influence of the two-phase flow (in the discharged flow structure) on the operating characteristics of SI is discussed.

A suite of methods has been established to quantitatively estimate uncertainties existed in source term analysis during a nuclear reactor severe accident. The accident sequence occurred at Unit 2 of the Fukushima Daiichi nuclear power plant (NPP) is taken as an example in which it is numerically modeled via the integrated severe accident code MELCOR 1.8.5. This standardized approach mainly consists of four steps: screening analysis, random sampling, numerical computation and verification of uncertainty distributions. First, by using an individually randomized one-factor-at-a-time screening method, a group of variables are preliminarily determined as important uncertain variables. Second, appropriate probability distributions are assigned to all selected variables. Multiple sets of random samples are generated using Latin Hypercube sampling combined with the consideration of rank correlation among input variables. Third, random samples of all selected variables are inputted into MELCOR 1.8.5. Numerical simulation with multiple code runs is implemented. Finally, uncertainty distributions for representative source terms (barium cesium, cesium iodide and tellurium) are obtained and verified. The technique of Bayesian nonparametric density estimation is applied to obtain probability density functions of interested source terms. In order to obtain a reasonable uncertainty distribution, several rounds of Latin Hypercube sampling and computation are conducted. As an alternative method to Wilks sampling criteria, the difference of probability density functions is evaluated through the comparison based on the Kullback-Leibler (KL) divergence. With the subjective judgment of small enough KL divergence, after a certain number of numerical computations, the uncertainty distributions of representative source terms are considered as stable enough as reliable results.

The Fukushima Daiichi Nuclear Power Plant accident and its consequences have led to some rethinking about the safety technologies used in boiling water reactors (BWRs). We have been developing the following various safe technologies: a passive water-cooling system, an infinite-time air-cooling system, a hydrogen explosion prevention system, and an operation support system to better deal with reactor accidents. These technologies are referred to as “inherently safe technologies”. The passive water-cooling system and infinite-time air-cooling system achieve core cooling without electricity. These systems are intended to cope with a long-term station blackout (SBO), such as that which occurred at the Fukushima facility. Both these cooling systems remove relatively high decay heat for the initial 10 days after an accident, and then the infinite-time air-cooling system is used alone to remove attenuated decay heat. The hydrogen explosion prevention system consists of a high-temperature resistant fuel cladding made of silicon-carbide (SiC) and a passive autocatalytic recombiner (PAR). The SiC cladding generates less hydrogen gas than the currently used zircaloy fuel cladding when core damage occurs, and the leaked hydrogen gas is recombined by the PAR. When a large-scale natural disaster occurs, fast event diagnosis and recognition of damaged equipment are necessary. Therefore, the operation support system consists of event identification and progress prediction functions to reduce the occurrence of false recognitions and human errors. This paper describes the following items: the targeted plant system; evaluation results on the required water pool capacity for 10-day water-cooling; development items for the water- and the air-cooling systems, the hydrogen explosion prevention system and the operation support system.

An experiment on a PWR station blackout transient with the TMLB' scenario and accident management (AM) measures was conducted using the rig of safety assessment/large scale test facility (ROSA/LSTF) at Japan Atomic Energy Agency under an assumption of non-condensable gas inflow to the primary system from accumulator (ACC) tanks. The TMLB' scenario involves prolonged complete loss of alternating current power and unavailability of turbine-driven auxiliary feedwater as well as malfunction of relief valves in primary system and steam generator (SG) secondary-side system. The AM measures considered in this study are SG secondary-side depressurization by fully opening the safety valves in both SGs with the start of core uncovery and coolant injection into the secondary-side of both SGs at low pressures. The LSTF test revealed that the primary pressure started to decrease when the SG primary-to-secondary heat removal resumed soon after the coolant injection into the SG secondary-side. The primary depressurization worsened due to the gas accumulation in the SG U-tubes after the completion of ACC coolant injection. The RELAP5 code well predicted the overall trend of the major phenomena observed in the LSTF test, and indicated remaining problems in the predictions of the SG U-tube collapsed liquid level and primary mass flow rate after the gas ingress. The SG coolant injection flow rate was found to significantly affect the peak cladding temperature and the ACC actuation time through the RELAP5 sensitivity analyses.

Thermal striping caused by the mixing of fluids at different temperatures is one of the most important issues in the design of Sodium cooled Fast Reactors (SFRs), because it may cause high-cycle thermal fatigue in the structure and affect the structural integrity. A numerical simulation code named MUGTHES has been developed to investigate thermal striping phenomena and to estimate high-cycle thermal fatigue in SFRs. In this study, the numerical simulation of the WATLON which was a water experiment of the T-junction piping system conducted by the Japan Atomic Energy Agency (JAEA) was conducted to validate the MUGTHES as a typical problem of thermal striping and to investigate the temperature fluctuation generation mechanism relating to the unsteady motion of large eddy structures. In the numerical simulation, an approach using the large eddy simulation (LES) with the standard Smagorinsky model was employed to simulate large scale eddy motions in the T-pipe. To quantify the uncertainty of the numerical results, the Grid Convergence Index (GCI) estimation was examined using two modified methods from the Roache’s GCI method described in the ASME V&V-20 guideline and the Eça-Hoekstra’s least square version GCI. The modified least square version GCI was named SLS-GCI (Simplified Least Square version GCI estimation method). Three mesh arrangements were employed to estimate the GCI value for uncertainty quantification in the validation process. Through the GCI estimation, it was found that the SLS-GCI method could successfully quantify the uncertainty of the numerical results. The numerical results suggested that the fine mesh arrangement in this study could improve the temperature distribution in the wake and that the thermal mixing phenomena in the T-pipe were caused by the mutual interaction of the necklace-shaped vortex around the wake from the front of the branch jet, the horseshoe-shaped vortex, and Karman’s vortex motions in the wake.

A subchannel void sensor (SCVS) was developed to measure the cross-sectional distribution of void fraction in a 5×5 heated rod bundle with o.d. 10 mm and heated length 2000 mm, and applied to a boiling two-phase flow experiment under the atmospheric pressure condition assuming at an accident or in a spent fuel pool in a boiling water reactor (BWR). The SCVS comprises 6-wire by 6-wire and 5-rod by 5-rod electrodes. The wire electrodes of 0.2 mm in diameter are arranged in lattice patterns between the rod bundle, while the electric conductance value in a region near one wire and another corresponds to local void fraction in the central-subchannel region. The local void fractions at 32 points (= 6×6-4) can be obtained as a cross-sectional distribution. The local void fractions near the rod surface at 100 points (= 4×25) can be also estimated by the conductance value in a region between one wire and one rod. The devised sensors are installed at five height levels along the axis to acquire two-phase flow behavior. A pair of SCVS is mounted at each level and placed 30 mm apart to estimate the one-dimensional phasic velocity distribution based on the cross-correlation analysis of both layers. The temporal resolution of void fraction measurement is 1600 frames (cross-sections) per second. The axial and radial power profile of the heated rod bundle are uniform, and eight pairs of sheath thermocouples are embedded on the heated rod to monitor its surface temperature distribution. The boiling two-phase flow experiment, which simulated a boil-off process, was conducted with the devised SCVS and experimental data was acquired under various inlet flow velocity, rod bundle power and inlet subcooling conditions. The experimental results were presented by the axial and cross-sectional distributions of void fraction, phasic velocity and bubble-chord length.

Cement is a practical material for constructing the geological disposal system of radioactive wastes. The dynamic behavior of both permeability change and dissolution process caused by a high pH groundwater was explained using a double porosity model assuming that each packed particle consists of the sphere-shaped aggregation of smaller particles. This model assumes two kinds of porosities between the particle clusters and between the particles, where the former porosity change mainly controls the permeability change of the bed, and the latter porosity change controls the diffusion of OH- ions inducing the dissolution of silica. The fundamental equations consist of a diffusion equation of spherical coordinates of OH- ions including the first-order reaction term and some equations describing the size changes of both the particles and the particle clusters with time. The change of over-all permeability of the packed bed is evaluated by Kozeny-Carman equation and the calculated radii of particle clusters. The calculated result well describes the experimental result of both permeability change and dissolution processes.

Electromagnetic (EM) pumps, which drive liquid metals by electromagnetic force for Fast Reactor, had been developed. Annular Linear Induction Pumps (ALIPs) are one of EM pumps. There are some reports in which a drop and a fluctuation of the developed pressure occurred near the top of the developed pressure and flow rate relation curve (P-Q curve). This phenomenon was reported by Gailitis, Kirillov and Araseki. They simulated it using two-dimensional (2D) code and reported that it was characterized by vortices, which were initiated by azimuthal non-uniformity of sodium flow velocity and/or magnetic field, in the liquid sodium flow. We have calculated the developed pressure of EM pump by 2D magnetohydrodynamics (MHD) code and designed an EM pump. It was found that the code simulated the developed pressure with high accuracy in normal operations. When the flow rate was lower than one in the top of P-Q curve, the developed pressure's drop and fluctuation occurred. The fluctuation would disturb the stable operation of the pump. For avoiding this phenomenon, the EM pump's design becomes sometimes too conservative. To evaluate the quantitative effect of this phenomenon and occurred conditions, we have developed a new three-dimensional(3D) MHD code. Clarification of these conditions and its phenomena will enable us to design a new structure or determine operation conditions. This paper presents the simulation results in terms of the generation of a developed pressure's drop and fluctuation occurrence. We used initial conditions which had azimuthal non-uniformity of sodium velocity to simulate vortices in the liquid sodium. Our 3D MHD code simulated the developed pressure's drop and fluctuation by vortices in radial and circumferential direction. It was confirmed that the vortices developed in the radial direction and then the vortices in circumferential direction developed.

Compressed and uncompressed crushable foam blocks were subjected to projectile impact at low velocity to evaluate and compare the foam resistance against the projectile impact. Among many crushable foam applications their energy absorbing characteristics play a prominent role to secure the structures against impacting objects. In the present work numerical simulations using finite element hydrocode LS-DYNA were first carried out to study the crushable foam behaviour when subjected to the impact by steel projectiles moving at an average velocity of 48 ms-1. A number of experiments were conducted to validate the numerical modelling. In the numerical analysis the foam properties were based upon previous work on crushable foam (Shah and Topa, 2014). It was found that the projectile impact on compressed foam, enhanced resistance against projectile penetration. Compressing the foam increases its density which results in an increase in the rate of energy absorption per unit time.

In this paper, we present an analytical solution for an infinite strip having an eccentric circular hole when the strip is subjected to in-plane bending at infinity. The analysis is based on the Papkovich-Neuber displacement potentials and the solution is obtained by a proper combination of harmonic function in integral forms and infinite series. The boundary conditions on both sides of the strip and around the hole are satisfied using the relations between the Cartesian and polar harmonics. The numerical results obtained are compared with those of FEM. A detailed stresses around the eccentric hole are illustrated for the various size of the eccentricity and the hole radius.

The effects of heating history on the sinterability of nickel powder compacts as a model material are examined from a view point of driving force and flow resistance, that is, the sintering stress and viscosity. Sinter-compression tests were conducted at different constant temperatures for cylindrical specimens, which underwent one-step or two-step heating. In the two-step heating, the specimens were heated up to a higher temperature and cooled down to the test temperatures. After the tests, the microstructure of each specimen was examined and compared with the change in sintering stress and viscosity. For the whole range of test temperatures, viscosity was increased by the two-step heating, which can be explained by the effects of time-hardening. On the other hand, for the lower range of test temperatures, the sintering stress was decreased by the two-step heating. This may be a result of the formation of large pores due to inhomogeneous shrinkage in the unstable state of the powder with small contact in the initial stage of sintering.

The essential work of fracture (EWF) is a key property in understanding the fracture resistance in polymer membranes. As such, it is a promising approach when investigating the fracture resistance of proton exchange membrane in fuel cells. The longevity of these membranes is crucial to the good function of the cell: the membranes have to sustain important variations in the surrounding temperature and humidity, possibly affecting their fracture resistance. This study investigated the essential work of fracture of such proton exchange membranes using a double-edge notch tensile test (DENT test). The tests were performed for different environmental conditions that were relevant to the conditions met by proton exchange membrane fuel cells. The results of the DENT tests strongly depend on the temperature and humidity; in particular the high temperature cases show a large increase of dissipated energy. Based on experimental results, a numerical model was developed and the numerical simulations of DENT tests were performed. The obtained results suggest that the shape factor of plastic zone, β, should be a function of the ligament length and the quadratic regression is appropriate to the calculation of EWFs when the temperature is near the glass transition temperature. The EWFs under ambient temperature (30 °C) conditions were found to be 18.4 kJ/m2 for 50 %RH and 21.5 kJ/m2 for 100 %RH. Those under high temperature (80 °C) conditions were found to be 48.0 kJ/m2 for 50 %RH and 56.4 kJ/m2 for 100 %RH.

We elucidated the diffusive-thermal instability of premixed flames with low unburned-gas temperature under the adiabatic and non-adiabatic conditions. Numerical calculations of two-dimensional unsteady reactive flows were performed, based on the diffusive-thermal model equation. Lewis numbers smaller than unity were adopted, and radiative heat loss was treated. As the unburned-gas temperature became lower, the growth rate decreased and the unstable range narrowed, which was due to the decrease of the burning velocity of a planar flame. As for the growth rate and unstable range normalized by the burning velocity of a planar flame, the former increased and the latter widened. This was due to the enlargement of Zeldovich numbers. Taking account of radiative heat loss, the normalized growth rate was large and the normalized unstable range was wide. This indicated that the heat loss had a pronounced influence on the diffusive-thermal instability of premixed flames with low unburned-gas temperature. Moreover, the cellular-shape flame fronts formed owing to diffusive-thermal instability. The burning velocity of a cellular flame normalized by that of a planar flame increased as the unburned-gas temperature became lower and the heat loss became greater. This was because of the enlargement of Zeldovich numbers and the pronounced influence of heat loss.

We numerically study the forced-oscillation-frequency responses on the three-dimensional thermal convection in a cubic cavity heated from one wall and chilled from its opposite wall in the non-gravitational field at vibrational Rayleigh number (the Rayleigh number based on the cavity's acceleration amplitude instead of the gravitational acceleration) Raη = 5.0×103 - 1.1×105, Plandtl number Pr = 7.1 (water) and non-dimensional forced-oscillation frequency ω = 1.0×100 - 1.0×103. The direction of the forced sinusoidal oscillation is parallel to the temperature gradient inside the cubic cavity. We especially focus upon the influences of both Raη and ω. As a result, five kinds of structures S2 (with a single roll), S4 (with a toroidal roll), S5 (with four roll), S6 (with four roll) and Sα (with six roll) appear in the tested ranges of Raη and ω. The Sα consists of a pair of trident currents, namely, three ascending streams and three matching descending streams in the cubic cavity. And, such flow structures are revealed in detail. Whenever it is not conductive but convective for ω < 5.0×10 2, convective motion always starts with the S4 from the rest at each forcing cycle. We find out the optimum frequency ω|K|max where the amplitude of a spatially-averaged kinetic energy K, which is defined by the difference between the maximum K and the minimum K over one forcing cycle, attains the maximum at each Raη. At ω = ω|K|max max, the flow structure is characterised by the S4. So, this fact suggests that the optimum frequency can be related with the S4. In addition, we show the occurrence condition for convection as a function of Raη and ω, and the boundary for the quasi-steady approximation which is permissible at ω ≲ 100 from a quantitative viewpoint.

We develop autonomous agents that fight with each other, inspired by human wrestling. For this purpose, we propose a coupled inverted pendula (CIP) framework in which: (a) the tips of two inverted pendula are linked by a connecting rod, (b) each pendulum is primarily stabilized by a proportional-derivative (PD) controller, (c) and each is additionally equipped with an intelligent controller. Based on this framework, we dynamically formulate an intelligent controller designed to store dynamical correspondence from initial states to final states of the CIP model, to receive state vectors of the model, and to output impulsive control forces to produce the desired final states of the model. By developing a quantized and reduced-order design for this controller, we obtain a practical control procedure based on an off-line learning method. We then conduct numerical simulations to investigate the individual performance of the intelligent controller, and we show that the performance can be improved by adding a delay element. The results show that the performance depends not only on the quantization resolution of the learning data but also on the delay time of the introduced element. Finally, we install the intelligent controllers into both pendula in the proposed framework in order to demonstrate autonomous competitive behavior between inverted pendula.

We have developed a smart energy harvester that generates electrical energy from multi-modal vibrations. The harvester consists of a digital processor and a piezoelectric sensor, which allows the application of a technical method to improve the energy-conversion efficiency. The method is implemented by measuring the vibration displacements, processing the data digitally, and adequately regulating electric switches. These operations are managed by a built-in digital processor. The driving power for the digital processor is satisfied with a part of the energy harvested from structural vibrations. Thus, the harvester operates flexibly with the digital processor to enhance electrical energy generation, and requires neither batteries nor an external power supply. We refer to the proposed device as a self-powered energy harvester. An advantage of digital processing is that observations by a Kalman filter can be used to estimate modal structural vibrations. In addition to reducing sensor noise, the digital filter extracts modal values from the measured displacement data. Here, we describe the basic configuration of the proposed harvester and demonstrate energy harvesting from multi-modal vibrations for a structure with 2 degrees of freedom (DOF). We assess the internal energy consumption of self-powered control devices, such as the digital processor and DC/DC converter. In addition, we show the robustness of the proposed harvester by conducting a harvesting experiment with electrical noise. The results demonstrate that the self-powered energy harvester generates more electrical energy from 2DOF vibrations than does a conventional harvester, and accurately operates under noisy conditions.

A new steering law of a pyramid configuration of four control moment gyroscopes (CMGs) is proposed to suppress radical motion of gimbals, reduce the gimbal angular displacement and level the biased ones of the four CMGs. Additionally analytical findings on the execution timing of the proposed method in relation to multiple attitude maneuvers are newly presented. Because major failure mode of the CMG is the defective lubrication of the spin bearings, resulting from being put on the excessive radial loads by radical motion of gimbals, the proposed method focuses the relation between the gimbal angular displacement of each CMG and the initial condition of gimbal angles. A suitable set of initial gimbal angles is selected using the defined evaluation function in an off-line preliminarily calculation. The evaluation function considers the manipulability of the upcoming maneuver related to the radical motion of gimbals and the biased gimbal angular displacements of four CMGs accumulated during the operational time. Simultaneously, the criteria for judgment and analytical findings on the execution timing of the proposed method in relation to multiple attitude maneuvers are newly presented, which consider the extra load by null motion and the accumulated gaps of gimbal angles between the initial condition and the terminal ones. The dynamics of a typical pyramid configuration of four single-gimbal CMGs was modeled and a numerical simulation in case the CMGs do not pass through the singularity was carried out. Numerical simulation confirmed that the proposed method not only keeps the manipulability greater but also levels the gimbal angular displacements of the CMGs without degrading attitude control.

A mass-spring system with a dynamic vibration absorber has a fixed point in its frequency response curves. A method of measuring mass using this property was proposed. It has an advantage that mass is estimated independently of the damping in the absorber. The developed system was characterized by using a phase-looked loop to achieve the tuning condition. However, because the actuator producing the harmonic force was mounted on the base, the reaction force was transmitted to the surrounding structure. It may cause a severe vibration of the surrounding structure. Therefore, it is necessary to prevent transmission of force to the surrounding structure in measuring. In this paper, a new type of mass measurement system is proposed which uses an inertial-mass vibrator to reduce the transmitted vibration. The tuning conditions are derived for both systems with a damped absorber and with an undamped absorber. The principles of measurement are described based on a mathematical model. An experimental apparatus is designed and fabricated. The efficacy of the measurement system with an inertial mass-type vibrator is studied both analytically and experimentally.

This paper presents an approach for modeling and disturbance compensation in multi-axis shaking table systems. Shaking table system is one of the facilities for seismic tests, where the accurate reproducibility of earthquake acceleration waveforms is essentially desired to evaluate the precise vibratory responses of specimen. However, the rotational motion due to overturning moment of specimen generates as a disturbance for the actuator control system, resulting in the lower reproducibility for the desired earthquake acceleration by deterioration of table motion performance. In order to compensate for the disturbance, at first, the simulator of shaking table including the specimen and control system is modeled using a multibody dynamics analysis including in a control CAD software. Based on the simulator, compensation signals that can cancel the disturbance are generated by an iterative learning control on the simulation, where the compensation signals are stored as time-series data. By using the compensation signals in the actual experiments, effects of the disturbance can be suppressed by the compensation signals in a feedforward control manner without repetitive actual excitations. The effectiveness of proposed control approach has been verified by experiments using a laboratory prototype.

Medical instruments are generally sterilized using high-pressure steam, ethylene oxide gas, gamma radiation, or electron beams. However, such conventional methods entail many shortcomings such as limitations of applicable materials, high temperature, long time, and high operating costs. Therefore, development of a safe, low-temperature, rapid, and inexpensive sterilization method has been anticipated. This study developed and assessed a plasma sterilization device that can generate effective chemical species necessary for sterilization. Its sterilization effectiveness was verified using biological indicators of Geobacillus stearothermophilus and Bacillus atrophaeus spores. The G. stearothermophilus and B. atrophaeus were sterilized, respectively, in 25 and 35 min with 5,400 ppm and in 35 min with 7,600 ppm of plasma-generated NOx, respectively, at around 25 °C. The main factor in sterilization of spores using the plasma was also assessed. In the conditions of atmospheric pressure air plasma, nitrogen oxide such as NO and NO2 were mainly generated. Morphological observations of spores exposed to the plasma and NO2 using scanning electron microscopy confirmed that nitrogen oxide perforated the spore coats. That spore coat perforation caused by nitrogen oxide is expected to be a main factor in sterilization. To remove the toxic gases used for sterilization, two types of methods were introduced into the developed devices. Bubbling in water could be reduced the concentration of residual NOx in the reaction chamber after sterilization to less than 50 ppm, but it remained higher than the limitation of environmental regulations. On the other hand, using the surface discharge system, we were able to reduce the residual NOx to less than 0.04 ppm, which is the limit of environmental regulations. This study presents the important techniques to improve infection prevention and public health.

It is necessary to examine comfortable vibrations and swinging motions used in the development of a human-machine interface. Although the present study was limited to the rocking motion of a rocking chair, it was confirmed that the waveform of a swinging motion used as a type of stimulation, the period of which is constant and the power spectrum of the amplitude envelope has the property of 1/f, makes human users comfortable. Next, the features of a swinging motion produced by a person for their enjoyment were examined. As a result, the power spectrum of the envelope of the measured acceleration waveform had the property of nearly 1/f, that is, it had a similar waveform feature that was evaluated as comfortable. In this study, the author focused on the period fluctuations of rhythms produced by humans for their own enjoyment. The rhythms occurring when walking comfortably, hopping on a pogo stick, bouncing on an exercise ball, swinging on a swing, and dribbling a basketball were measured using a portable acceleration measurement system or a sound recording system. Then, the periods of the rhythms were obtained, and the power spectra were examined. As a result, the power spectra showed a downward slope within a low-frequency range; that is, they were similar to the property of 1/f. Further, it was clarified that these power spectra have about the same magnitude, the periods are about the same, and the period fluctuations are not very large.